Typical integrated circuit motor drive chips utilized to drive direct current motors with a pulse width modulated signal include two FETs. The FETs are operated such that when one FET switches on, the other FET switches off. The FET that switches on draws current from the voltage source through the motor winding to ground. The motor behaves as a mutually coupled inductance. The result is that a high reverse polarity voltage is applied to the FET that has switched off.
Each FET has an internal parasitic body diode across its drain-source. When a reverse polarity voltage is applied to the drain that exceeds the voltage drop of the body diode, the body diode becomes forward biased and a large current flows or is “injected” into the substrate. As the one FET draws current from the voltage source through its motor winding to ground, the body diode associated with the other FET causes substrate injection to occur.
One prior solution to the substrate injection problem is to utilize an external Schottky diode across each FET. The external Schottky diode is selected to have a lower voltage drop than the internal parasitic body diode of the FET. When a voltage spike occurs across a FET, the Schottky diode conducts thereby clamping the voltage across the FET to a voltage lower than that which will forward bias the body diode and preventing current injection into the substrate.
It is desirable to provide a structure in which injection of substrate current is prevented and that does not require use of an external diode clamp.